ISOFileSystem: Don't crash when the image has no ISO9660 volume

The constructor leaves treeroot null when it can't find a CD001 volume
descriptor, but GetFromPath walked into it anyway - TreeEntry *entry = treeroot;
then entry->valid - so any path lookup on a failed mount dereferenced null.

Reachable from the firmware installer, which mounts whatever file it's handed
and asks for PSP_GAME/SYSDIR/UPDATE without consulting Error() first. Point it
at a PlayStation disc image, whose descriptor sits behind a Mode 2 subheader and
so fails the signature check, and PPSSPP goes down. Identify_File checks for
CD001 before reporting PSP_ISO, so the game browser was never exposed.

Return null instead, which is what the rest of the function already does for a
path that isn't there, and what every caller expects.

Core/FileSystems/ISOFileSystem.cpp

@@ -335,6 +335,11 @@ const ISOFileSystem::TreeEntry *ISOFileSystem::GetFromPath(std::string_view path
 	if (pathLength <= pathIndex)
 		return treeroot;
 
+	if (!treeroot) {
+		// The constructor gave up - no ISO9660 volume descriptor, or it wouldn't read.
+		return nullptr;
+	}
+
 	TreeEntry *entry = treeroot;
 	while (true) {
 		if (!entry->valid) {